The invention relates to a filtering device for high-viscosity fluids.
Filtering devices designed as so-called “screen piston changers” are known for filtering high-viscosity media such as plastic melts in particular. In the design known from EP 1 778 379 B1, for example, two piston-like screen carrier elements are arranged movably in a housing. Each of these screen carrier elements has at least one screen location where a screen space is formed. At least one filter element is placed in each screen space. Feed channels in the housing branch off towards the screen spaces such that melt is guided through the respective filter elements into the screen spaces. Provided behind the filter element, when viewed in the direction of flow, are partial channels through which the plastic melt is discharged. The partial channels are combined at some point at or in the housing or in a joint discharge channel that runs out of the housing.
The known filtering device also allows for backwashing of the filter elements by moving one respective screen carrier element out of the production position such that fluid or melt no longer arrives at the dirt side of the screen. Melt is guided from the rear side, the so-called clean side, into the screen space and from the rear side through the filter element. Particles or agglomerates adhering to the filter element on the dirt side can be loosened from the filter element through the backwash procedure.
In the device disclosed in EP 1 778 379 B1, the basic principle of a backwashable screen piston changer is improved such that displacing pistons are provided for each partial channel that leads away from a screen location and where said displacing pistons can dip into the partial channels on the clean side. A blockage of the discharge channel is effected at the moment when the displacing pistons are inserted into the partial channels. Melt that is present in the partial channel is thus no longer pressed into the discharge channel and therefore into the subsequent unit but exclusively into the screen location provided for backwashing. The displacing piston pushes the fluid located in the partial channel in the flow direction opposite to the one during production operation into the screen space and from behind through the filter element in order to loosen build-up on said filter element.
The advantage of the additional displacing piston is that filtering in the respective other screen carrier element and/or at the other screen locations is completely unaffected by the backwash procedure. While traditional backwash screen piston changers always had to divert a portion of the fluid from the production operation is order to carry out the backwash, the displacing pistons allow for decoupling of the pressure and of the fluid amount required for the backwash from the production process. The only connection between the screen locations on the dirt side is the joining point of the partial channels on the dirt side or the discharge channels that lead from both screen locations to a joint emptying point at the housing. By the fact that the displacing piston blocks this discharge channel during backwash, the connection to the respective other flow path is interrupted as well. There is no pressure loss in the production strand during the backwash.
Thus, the known filtering device with the displacing piston has proven itself. However, a disadvantage is that a larger fluid volume must be kept available in the area between the screen location and the displacing piston, in order to perform later on an effective backwash using the displacing piston. An enlarged diameter must therefore be provided in the partial channel in order to keep a larger volume temporarily available for the given size and thus the limited length of the partial channel. This change in the flow channel geometry on the discharge side in turn leads to reduced flow velocities and thus to longer residence times of the fluid amount present in the partial channel, which may lead to the risk of thermally induced changes during the filtration of the plastic melt in connection with the heated housing, for example.